Genetics: Key Facts
Genes and Nucleic Acids
• The DNA molecule and other nucleic acids are composed of nucleotides.
Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous
base. DNA has four different bases.
• The structure of DNA resembles a spiral staircase of two intertwining
strands. Each strand consists of a covalently bonded backbone composed of
phosphate groups alternating with the sugar deoxyribose.
• The nitrogenous bases form the stepwise connections between the two
backbones, which are held together by hydrogen bonds.
• DNA is complexed with proteins to form a filamentous material called
chromatin. Human chromatin consists of 46 long filaments called
chromosomes, which come in two sets of 23 with each set from each parent.
• Ribonucleic acid (RNA) is produced in three types: messenger RNA,
ribosomal RNA, and transfer RNA. RNA and DNA have significant
differences although both are nucleic acids.
• RNA is a single nucleotide chain, not a double helix. The sugar in RNA is
ribose, not deoxyribose; and one RNA base is different from the one in
DNA. The essential function of RNA is to interpret the code in DNA and
direct the synthesis of proteins.
DNA Replication and the Cell Cycle
• DNA replication is necessary prior to cell division, and it is accomplished
through complementary base pairing.
• Cells divide by two mechanisms: mitosis and meiosis. Meiosis is restricted
to production of eggs and sperm, and mitosis serves all